This invention relates to improved apparatus and methods for detecting a selected chemical species, hereinafter referred to as xe2x80x9canalyte.xe2x80x9d My invention concerns sensor devices, advances in analytical capability, and new applications of chemical sensing technology. Although the invention is directed primarily to the detection of gaseous analytes, some of its novel aspects may also apply to liquid and even solid substances.
While the present invention can have broad implications for the improvement of various types of analytical sensors, its focus is on the amperometric gas sensor, hereinafter referred to as xe2x80x9cAGSxe2x80x9d.
The AGS has been in existence since the Clark electrode of the 50""s was developed for measurement of oxygen in blood. The modern carbon monoxide [CO] sensor has existed since 1969 and resulted from the novel application of the Teflon-bonded diffusion electrode to the measurement of alcohol [ethanol] in the breath and of CO in ambient air. Subsequent improvements yielded: [a] smaller sensors, with even some micro-fabricated versions reported; [b] wick or matrix electrolytes with improved lifetime and reduced attitude-sensitivity; [c] applicability to a number of new analytes besides O2, ethanol, and CO; and [d] more cost-effective manufacture. Also, during the last 30 years, the AGS has become increasingly important in industrial, medical, and environmental applications and has become one of the most successful and widely used chemical sensors.
The AGS is useful in numerous applications, such as medical oxygen measurement, environmental analysis, or toxic gas detection, including home CO alarms and personal toxic gas alarms used to protect human health and the environment on a daily basis. The gases that have been most significant commercially in the repertoire of the AGS include but are not limited to CO, oxygen, H2S, NO, NO2, SO2, monomethylhydrazine, ethanol, and many others. Table 1 gives a few examples of the electrochemical reactions that have been proposed for some typical analytes. There are commercially available AGSs for many of these analytes and many more are possible. High temperature versions of the AGS using solid electrolytes further expand the applications of these sensors to automotive uses.
Today, such sensors have typical sensitivities in the range of parts per million [ppm] to parts per billion [ppb] by volume. This is the typical limit of detection for simple chemical sensors. Such sensors usually comprise: [a] a working electrode [WE] at which the analyte gets consumed by a half-cell reaction such as those listed in Table 1; [b] a counter electrode [CE] for a complementary half-cell reaction; and [c] a reference electrode [RE], which serves to control the electrochemical potential of the WE, and all three electrodes are in electrolytic contact, with the WE designed to be exposed to the analyte-containing gas sample. The CE and RE are sometimes combined to form a single counter and reference electrode, CE/RE. The term xe2x80x9cauxiliary electrodexe2x80x9d is sometimes used to refer to the CE or CE/RE. Present limitations to sensitivity and selectivity are tied to signal size, noise, drift, and background current of the sensor and ultimately to the choice of: 1) materials used for electrocatalysts [electrodes] and electrolytes, and 2) structure [geometry] and methods of operation of the sensor. Most of the efforts to-date at improving the sensitivity and selectivity of an AGS were focused on the composition of the WE and on its geometrical structure and arrangement relative to the RE and CE.
To reduce the detection limit of an AGS to much lower analyte concentrations, e.g., to as low a range as parts per quadrillion [ppq] by volume, my invention focuses on the composition, structure, relative arrangement and operation of the CE, and/or the RE, so as to: a) greatly enhance the selectivity of an AGS to specific analytes by minimizing spurious signals from interfering species; and b) increase its sensitivity by orders of magnitude through amplification of signals due to any selected analyte.
The closest publication akin to my amplification concept is that of F. R. Fan and A. J. Bard, in Science, Volume 277, Pages 1791-1793, 1997, wherein a redox molecule is trapped and cycled in a tiny volume of liquid. Other partly relevant work involves the use of more than one electrode in capillary electrophoresis [F.-M. Matysik et al., xe2x80x9cApplication of microband array electrodes for end-column electrochemical detection in capillary electrophoresis,xe2x80x9d Analytica Chimica Acta, 385, 409-415, 1999] and in other all-liquid systems [O. Niwa et al., Electrochemical Behavior of Reversible Redox Species at Interdigitated Array Electrodes with Different Geometries: Consideration of Redox Cycling and Collection Efficiency,xe2x80x9d Anal. Chem., 62 447-452, 1990], wherein the first electrode does oxidation and the second does reduction to get selectivity improvements; but these are akin to the rotating ring disk situation for electrochemical studies wherein selectivity or reaction products are confirmed. None of these prior publications includes repeated amplification of a signal from an analyte in a gaseous medium, as described herein.
It is an object of my invention to effectuate improvements in gas detection wherever additional sensitivity and/or better selectivity may be required.
A significant improvement in the AGS should benefit not only its existing applications but also make possible many new uses. Specific new benefits could range from improving the treatment for neonatal jaundice to discovering unexploded ordnance or contraband drugs. It is therefore an object of my invention to provide a sensor with femtomolar sensitivity and high selectivity, which can be micro-fabricated and is inexpensive, robust, and consuming minimal power, for many applications in medicine, environment, and industry, including automotive uses.
It is an object of my invention to provide radically new sensor designs that will advance the science and art of chemical sensing, including electrochemistry, amperometry, catalysis of gas phase electrochemical reactions, and chemical sensors, in a significant manner.
An object of this invention is to utilize new sensor structures and geometries to achieve million-fold or larger improvements in the analytical sensitivity of the AGS.
Another goal of my invention is to provide a simple and inexpensive xe2x80x9cchemically-amplifiedxe2x80x9d AGS.
An object of the invention is to create chemical sensors that can amplify a minute change so that low concentrations of analyte can be detected by a single tiny chemical sensor.
These objectives are further tied to industrial interests by their applicability to NOx sensing and their potential to vastly improve sensor specifications for automotive uses.
It is another object of my invention to provide better sensors leading to improved analytical instruments which will impact various fields of science and engineering with cost-effective analytical tools for medical, environmental, and materials research, development, and applications.
In biological areas, NO is important in the study of neuron function and CO in the breath of neonates is an indicator of jaundice. Current techniques are expensive or cumbersome, and a simple and selective AGS for these compounds would provide significant benefit. It is therefore a specific object of my invention to provide ultra-sensitive AGSs having the ability to measure trace concentrations of NO and CO.
Current CO sensors are not quite adequate for fuel cells, home CO alarms, and micro-noses, and a better sensor would benefit society. Gas sensors are becoming more commonplace in industry and society for safety, health, and environmental measurement and control. Gas sensors in the long run will become widely used in many more places, such as elevators, buildings, cars, homes and the like, to provide for improved human health, safety, and comfort. Gas sensors will lead to better process and products instrumentation and control. Critical applications like humanitarian de-mining or detection of explosives at airports are not possible with present chemical sensors, indeed with any sensors or analytical instruments, and still rely on the imperfect but elegantly sensitive dog""s nose. In order to rival such sensitivity, an improvement of several orders of magnitude is required in the present AGS. It is therefore still another object of my invention to provide detectors that mimic the dog""s nose and are millions of times more sensitive than current systems.
The present CO sensor is difficult to use for stack gases and fuel cell automotive vehicles because it has cross sensitivity to many other pollutants and cannot operate effectively above 60xc2x0 C. The present CO sensor lacks ability to be used on very small samples with high sensitivity and stability and cannot be easily used in the treatment of jaundice in newborn infants. Therefore, it is an object of my invention to provide an AGS with improved sensitivity and selectivity for CO that could make these applications possible.
It is yet a further object of my invention to provide an improved NO2 and NO sensor that is inexpensive and of small size and weight, has extremely high sensitivity, and consumes little power, that could contribute to better physiological measurements of NO or to better measurements of NO and NO2 in environmental and automotive applications.
It is yet another object of my invention to provide an improved AGS which can yield analytical devices that are portable, inexpensive, and have the potential for microfabrication.
More objects of the invention will become apparent to professionals in the industrial safety and hygiene, environmental, law enforcement, health monitoring, chemical, metallurgical, and related areas following perusal of the complete specification.
A key feature of my invention is that it focuses primarily on the CE and/or RE rather than on the WE, in distinction from what was done in most previous advances in AGS technology.
Briefly, my invention effectuates major improvements in the AGS through the following modifications:
1. Adjustments in structure, composition, and electrode potential which result in minimal or no reactivity of any analyte crossing over to the CE and/or RE, preferably combined with maximal reactivity of the CE to the product of the analyte reaction at the WE; and
2. A structural arrangement of the CE relative to the WE whereby the product of the analyte reaction at the WE can be reconverted to the analyte at the CE and then reacted again at the WE, with such back-and-forth reactions repeating many times, so as to yield an amplification of the analyte signal; or
3. Alternatively, a structural arrangement whereby said reconversion of the analyte reaction product occurs at a second WE rather than at a CE.